1. Field of the Invention
The present invention relates to well gravel-packing methods, and more particularly, to improved methods of forming gravel packs in vertical and nonvertical wellbores.
2. Description of the Prior Art
The gravel packing of wellbores and perforations extending therefrom into subterranean producing zones has been practiced in the oil field for many years. Such gravel packing involves the placement of a tightly packed mass of particulate solid material in the wellbore and perforations extending therefrom so that loose or incompetent subterranean formation materials produced with hydrocarbons are screened out by the gravel pack and are prevented from entering the wellbore.
A gravel pack is typically formed in a wellbore by placing a tubular gravel pack screen in the wellbore adjacent the perforations therein and then injecting a carrier liquid having a particulate solid pack material suspended therein into the space between the exterior of the screen and the walls of the wellbore containing the perforations. The pack material which has heretofore typically been sand or bauxite, is screened out of the carrier liquid and a pack of the material is formed in the perforations and in the annular space between the screen and the wellbore walls. An alternate technique involves injecting a carrier liquid-pack material suspension into the wellbore and into the perforations whereby the perforations are packed and then setting the screen and packing the annulus using the same or different carrier liquid-pack material suspension.
A problem which has continuously been associated with gravel packing procedures, particularly in nonvertical wellbores, is that the pack material settles out of the carrier liquid and does not enter one or more of the perforations. This results in unpacked perforations and voids in the gravel pack which allows the production of fines and sand with produced fluids when the well is placed on production.
In horizontal wells, i.e., wells that are drilled and completed with the portion of the wellbore in the producing formation or zone positioned substantially horizontally, the perforated horizontal portion of the wellbore can be very long. In gravel packing such horizontal wellbore sections, the pack material often settles to the bottom of the horizontal wellbore as the carrier liquid-pack material suspension flows therethrough resulting in voids in the gravel pack as well as unpacked perforations positioned in the top of the wellbore.
It is known that the transport of pack material such as sand without settling over a long nonvertical distance requires either a viscosified carrier liquid or very large volumes of a low viscosity carrier liquid, e.g., brine. High viscosity carrier liquids having sand suspended therein have not provided the degree of perforation and annulus packing needed to prevent gravel pack voids. On the other hand, low viscosity carrier liquid suspensions of sand have shown good packing efficiency in horizontal wellbores, but the sand loading of the suspension must be low which results in large volumes of the carrier liquid entering the production zone by way of the perforations. This in turn often results in considerable damage to the producing zone as a result of, for example, the swelling or migration of formation clays and fine material.
Thus, there is a need for an improved gravel packing method featuring the use of a low viscosity carrier liquid, such as brine, containing a high loading of pack material. Such a carrier liquid-pack material suspension would allow the pack material to be transported long distances in wellbores without settling, would form tight and uniform gravel packs and would limit fluid lost into the formations to thereby minimize damage to producing formations.